The present invention relates to a lock-up torque converter and a method for controlling the clutch slip in a lock-up torque converter.
Torque converters which provide a hydrodynamic drive produce poor power transmission efficiency although they function to absorb the variations in torque, thus providing a smooth power transmission. Lock-up torque converters have been proposed to improve the power transmission efficiency of the torque converters. The lock-up torque converters have a fluid operated friction clutch which is often called as a "lock-up clutch" and provide a hydrodynamic drive when the clutch is disengaged and a mechanical drive when the clutch is engaged. Although the power transmission efficiency is increased if the clutch is left engaged, the lock-up torque converter tends to vibrate under certain operating conditions of a prime mover if the clutch is engaged.
Laid-open Japanese Patent Application No. 57-33253 discloses a lock-up torque converter wherein clutch slip is detected by measuring a difference between rotational speed of a crank shaft of an engine and that of an output shaft of the torque converter and the engagement of a fluid operated clutch is adjusted to a degree where the measured difference agrees with a preset value. Since in this known lock-up torque converter, a change in the clutch slip resulting from a chnage in the output torque of the prime mover is detected after the occurrence of the change in the clutch slip, the degree of the engagement of the clutch slip which is effected in response to the change in the clutch slip tends to be always delayed, thus failing to accomplish satisfactory accuracy in control. Owing to the tolerances upon manufacture, the lock-up torque converters do not have uniform torque transmitting characteristics. Thus, the degree of clutch slip desired for a transmission torque which depends on the torque transmitting characteristic varies from torque converter to torque converter. As a result, since the preset value to which the clutch slip is adjusted cannot be adjusted to the variation in the torque transmitting characteristic owing to the tolerance, a desired result intended by the control can not be obtained throughout the manufactured torque converters. Thus, a deviation in control has resulted.
U.S. Pat. No. 4,002,228 discloses a lock-up torque converter wherein a hydrodynamic drive and a fluid operated clutch drive are arranged in parallel drive relationship. The clutch slip is controlled by a passage having a variable opening responsive to the torque transmitted by the hydrodynamic drive to split the input torque so as to limit the torque transmission through the hydrodynamic drive. The clutch transmits the remainder of input torque from the prime mover to a mechanical gear arrangement. Since the clutch slip is controlled in response to the torque transmitted by the hydrodynamic drive only and thus the clutch slip is adjusted after vibration resulting from a change in the torque transmitted has taken place, it cannot prevent the occurrence of vibration resulting from the change in the torque transmitted by the hydrodynamic drive during a portion of operating conditions of the vehicle, such as upon acceleration.